This invention relates to a photocatalytic body having a good photocatalytic function, a method for making the same, and a photocatalytic composition used therefor.
When semiconductors are irradiated with light whose wavelength has an energy greater than a band gap thereof, an oxidation-reduction reaction is brought about. Such a semiconductor is called a photocatalytic semiconductor or merely a photocatalyst.
Photocatalysts are in the form of powder and may be used as suspended in a solution, or may be used as supported on a substrate. From the standpoint of photocatalytic activity, the former is more active owing to the greater surface area. From the standpoint of practical applications, it has been frequently experienced to inevitably adopt the latter rather than the former owing to the ease in handing.
In order to support a photocatalyst on a substrate, there has been adopted a method wherein the particles of a photocatalyst are sintered at high temperatures and supported on the substrate. Another method has been proposed wherein a certain type of fluoropolymer is used as a binder, with which a photocatalyst is supported on a substrate. For instance, Japanese Laid-open Patent Application No. 4-284851 sets out a method wherein a mixture of the particles of a photocatalyst and a fluoropolymer is built up as layers and bonded under compression pressure. Japanese Laid-open Patent Application No. 4-334552 sets forth a method wherein the particles of a photocatalyst are thermally bonded to a fluoropolymer. Moreover, Japanese Laid-open Patent Application No. 7-171408 sets out a method wherein the particles of a photocatalyst is bonded on a substrate through a hard-to-decompose binder including an inorganic binder such as water glass or an organic binder such as a silicone copolymer, and also a method for manufacturing a photocatalytic body which includes, on a substrate, a first layer made of a hard-to-decompose binder, and a second layer formed on the first layer and made of a hard-to-decompose binder and the particles of a photocatalyst. In addition, Japanese Laid-open Patent Application No. 5-309267 describes a method wherein the metal oxide obtained from a metal oxide sol is used to support and fix the powder of a photocatalyst therewith. It is stated that the metal oxide sols are obtained from organometallic compounds such as alkoxides, acetylacetonate, carboxylates of metals as used in a sol-gel method, or are obtained by hydrolysis of an alcohol solution of chlorides, such as titanium tetrachloride, in the presence of an acid or alkali catalyst.
In recent years, attempts have been made to decompose, purify and sterilize harmful substances, offensive odor components and oily components ascribed to daily living environments by use of photocatalysts, thus leading to a quick extension of the application range of photocatalysts. This, in turn, requires a method of causing the particles of a photocatalyst to be firmly supported on all types of substrates over a long time without a sacrifice of its photocatalytic function. Especially, where a titanium oxide sol, which exhibits the good photocatalytic function but is poor in the function of bonding to a substrate, is used as a photocatalyst, it is required to improve the bonding property.
However, in these prior art methods, the bonding strength is not satisfactory, so that few methods ensures the support over a long time. If it is intended to make a photocatalytic body which has an improved bonding strength and ensures the support over a long time, there has arisen the problem that the photocatalytic function lowers. In case where the substrate made of an organic polymer resin is employed and rutile titanium oxide, which is weaker in photocatalytic function than anatase titanium oxide, is used, the photocatalytic reaction proceeds. Not only the organic polymer resin per se undergoes a photochemical reaction, but also the use over a long time results in degradation and decomposition.
Moreover, where organic polymer resins are used as a substrate, preliminary coating such as with a silica sol has been attempted, with the attendant problem that during the course of coagulation drying of the silica sol, cracks or voids are formed, thus presenting a problem on their bonding performance.
In order to solve the above problems, studies have been made on how to firmly support the particles of a photocatalyst on all types of substrates over a long time without impeding its photocatalytic function. As a result, it has been unexpectedly found that when using an amorphous titanium peroxide sol as a binder, the particles of a photocatalyst can be firmly supported on all types of substrates over a long time without impeding the photocatalytic function. The invention has been accomplished based on the finding.
More particularly, the invention relates to a method for manufacturing a photocatalytic body by use of a photocatalyst such as of titanium oxide and an amorphous titanium peroxide sol so that the photocatalyst is fixedly supported on a substrate, and also to a method for manufacturing a photocatalytic body which comprises forming, on a substrate, a first layer of an amorphous titanium peroxide sol having no photocatalytic function, and further forming a second layer on the first layer wherein the second layer is made of a photocatalyst and an amorphous titanium peroxide sol. Further, the invention relates to a photocatalytic body obtained by these methods and to a photocatalyst composition used for the manufacture.
The amorphous titanium peroxide sol used in the practice of the invention may be prepared, for example, by the following manner. An alkali hydroxide such as aqueous ammonia or sodium hydroxide is added to an aqueous solution of a titanium salt such as titanium tetrachloride, TiCl4. The resultant light bluish white, amorphous titanium hydroxide, Ti(OH)4, may be called ortho-titanic acid, H4TiO4. This titanium hydroxide is washed and separated, after which it is treated with an aqueous hydrogen peroxide solution to obtain an amorphous titanium peroxide solution useful in the present invention. The amorphous titanium peroxide sol has a pH of 6.0xcx9c7.0 and a particle size of 8xcx9c20 nm, with its appearance being in the form of a yellow transparent liquid. The sol is stable when stored at normal temperatures over a long time. The sol concentration is usually adjusted to a level of 1.40xcx9c1.60%. If necessary, the concentration may be optionally controlled. If the sol is used at low concentrations, it is used by dilution such as with distilled water.
The amorphous titanium peroxide sol remains as amorphous and is not crystallized in the form of anatase titanium oxide at normal temperatures. The sol has good adherence, a good film-forming property and is able to form a uniform flat thin film, and a dried film has such a property of being insoluble in water.
It will be noted that when the amorphous titanium peroxide sol is heated to 100xc2x0 C. or above, it is converted to anatase titanium oxide sol. The amorphous titanium peroxide sol, which has been dried and fixed on a substrate after coating, is converted to anatase titanium oxide when heated to 250xc2x0 C. or above.
The photocatalysts usable in the present invention include TiO2, ZnO, SrTiO3, CdS, CdO, CaP, InP, In2O3, CaAs, BaTiO3, K2NbO3, Fe2O3, Ta2O5, WO3, SaO2, Bi2O3, NiO, Cu2O, SiC, SiO2, MoS2, MoS3, InPb, RuO2, CeO2 and the like. Of these, titanium oxide is preferred. Titanium oxide may be used in the form of particles or powder, or in the form of a sol.
Titanium oxide in the form of a sol, i.e. a titanium oxide sol, can be prepared by heating an amorphous titanium peroxide sol at a temperature of 100xc2x0 C. or above. The properties of the titanium oxide sol, more or less, change depending on the heating temperature and the heating time. For instance, an anatase titanium oxide sol which is formed by treatment at 100xc2x0 C. for 6 hours has a pH of 7.5xcx9c9.5 and a particle size of 8xcx9c20 nm, with its appearance being in the form of a yellow suspension.
The titanium oxide sol is stable when stored at normal temperatures over a long time and may form a precipitate on mixing with an acid or a metal aqueous solution. Moreover, the sol may be impeded in its photocatalytic activity or an acid resistance when Na ions co-exists. The sol concentration is usually adjusted to a level of 2.70xcx9c2.90% and may be employed after adjustment of the concentration, if necessary.
A titanium oxide sol is preferably used as a photocatalyst. Commercially available xe2x80x9cST-01xe2x80x9d (ISHIHARA SANGYOU KAISHA Ltd) or xe2x80x9cST-31xe2x80x9d (ISHIHARA SANGYOU KAISHA Ltd) may also be usable.
In the practice of the invention, the substrate used may be made of inorganic materials such as ceramics, glass and the like, organic materials such as plastics, rubber, wood, paper and the like, and metals such as aluminium, steels and the like. Of these, applications to organic polymer resin materials, such as acrylonitrile resin, vinyl chloride resin, polycarbonate resins, methyl methacrylate resin (acrylic resins), polyester resins, polyurethane resins and the like, show good effects. The substrate is not critical with respect to the size or shape and may be in the form of a honeycomb, fibers, a filter sheet, a bead, a foamed body or combinations thereof. If a substrate which allows transmission of UV light is used, a photocatalytic body may be applied to the inner surface of the substrate. The body may also be applicable to coated articles.
In the present invention, binders which are incapable of being decomposed with a photocatalyst mean those binders incapable of being decomposed with photocatalysts and composed of inorganic binders such as water glass, colloidal silica, cement and the like, and organic binders such as fluoropolymers, silicone polymers and the like, as disclosed in the aforementioned JP-A7-171408.
The composition used to make a photocatalytic body may be prepared according to several methods.
One of such methods includes the use of a uniform suspension of titanium oxide powder in an amorphous titanium peroxide sol. For the uniform suspension, it is advantageous to employ ultrasonic wave after mechanical agitation.
Next, the titanium oxide sol and the amorphous titanium oxide sol are mixed to obtain a mixed sol. The mixing ratio is determined depending on the portion of a product to which a photocatalytic body is applied and the use conditions of an instrument using the body. For the mixing, consideration should be taken to the adherence to a substrate, film-forming properties, corrosion resistance and decorativeness of the photocatalytic body made by use of the mixed sol. The mixing ratio can be properly determined depending on the types of articles to be applied which are broadly classified into the following three groups.
(1) Those articles which one contacts or is highly likely to contact and which need decorativeness from the visual standpoint, e.g. interior tiles, sanitary wares, various types of unit articles, tablewares, exterior materials in buildings, interior automotive trims and the like.
(2) Those articles which one does not contact but requires visual decorativeness, e.g. exterior panels for light fittings, underground passage, tunnel, materials for engineering works, and electrical equipments.
(3) Those articles which one does not usually contact or is able to see and in which the function of decomposing organic matters based on a photocatalytic function or the properties inherent to semiconductive metals are utilized, e.g. built-in members in the inside of water-purifier tanks, various types of sewage treatment equipments, water heaters, bath tubs, air conditioners, the hoods of microwave ovens, and other apparatus.
For Group (1), a photocatalytic body which is obtained, in the form of a film, from a mixed sol wherein the titanium oxide sol is mixed in an amount of 30 wt % or below based on the total of the titanium oxide sol and an amorphous titanium peroxide sol is preferred. It has been found that articles using the body are sufficient for sterilization or decontamination in daily life and also for decomposition of residual odors. Moreover, the film surface is so hard that it is free of any wear such as by sweeping or dusting and also of any deposition of foreign matters, along with the unlikelihood of leaving fingerprints on contact.
With water-purifier tanks which belong to Group (3), for example, high photocatalytic activity is the most important property which is required for the photocatalytic body in order to lower a biological oxygen demand (BOD) in final waste water-treated water. It has been found that a photocatalytic body in the form of a film, which is formed of a mixed sol wherein the titanium oxide sol is mixed in an amount of 70 wt % or above based on the total of the titanium oxide sol and the amorphous titanium peroxide sol, is most suitable for this purpose. This photocatalytic body is poor in decorativeness. Since the articles of this group are ones which do neither come in contact with men nor are fell on the eyes. Moreover, it has also been found that such a problem of deposition of a residue in a slight degree can be solved by periodic removal and cleaning.
For the articles of Group (2), it has been found that a photocatalytic body in the form of a film, which is formed by use of a mixed sol wherein the titanium oxide sol is mixed in an amount of 20xcx9c80 wt % based on the total of the titanium oxide sol and an amorphous titanium peroxide sol, is suited. This photocatalytic body exhibits properties intermediate between the former two bodies with respect to the hardness, the adherence of foreign matters, and the photocatalytic activity.
For the coating or spraying, on a substrate, of a titanium oxide sol, an amorphous titanium peroxide sol or a mixed sol, any known procedures may be utilized including, for example, dipping, spraying, coating and the like. Good results of coating are frequently obtained when repeating the coating step plural times.
After coating or spraying as mentioned above, the sol is dried and solidified to obtain a photocatalytic body of the invention. The sol may be baked at approximately 200xcx9c400xc2x0 C. and fixedly set on a substrate. The photocatalytic function of titanium oxide lowers by the action of sodium ions. Accordingly, if an organic polymer resin which is liable to undergo decomposition by means of a photocatalyst is used as a substrate, it is preferred to clean the resin surface with a sodium ion-containing material such as a sodium hydroxide solution to permit a sodium source to be present.
It will be noted that where an amorphous titanium peroxide sol is used as a first layer, the peroxide is converted to the crystals of anatase titanium oxide on heating to 250xc2x0 C. or above, thereby causing a photocatalytic function to develop. Accordingly, lower temperatures, for example, of 80xc2x0 C. or below are used for drying and solidification. In this case, sodium ions may be added to the titanium peroxide sol for the reasons set out above.
Prior to shaping, the particles made of a spontaneous UV radiating material or a light storage-type UV radiating material, or particles containing such radiating materials may be mixed with a photocatalyst.
The spontaneous UV radiating material (i.e. a spontaneous light-emitting ceramic) is able to emit light by consumption of its internal energy, and makes use of radioactive disintegration of radium or promethium. The emitted light is within a UV range. In practice, a purified powder of rock containing such a component as mentioned above is set into a massive body, and the particles obtained by pulverization of the massive body into pieces are used.
The light storage-type UV radiating material (a light storage-type light emitting ceramic) is one which takes an external energy therein and emits light while releasing once taken energy. The emitted light is within a UV range. Such a material is commercially available under the designations of xe2x80x9cLumiNovaxe2x80x9d (commercial name of NEMOTO and CO., LTD) and xe2x80x9cKEPRUSxe2x80x9d (commercial name of Next xe2x80xa2 I CO., LTD). These are made primarily of strontium aluminate (SrAl2O4) containing highly pure components such as alumina, strontium carbonate, europium, dysprosium and the like. The maximum point of the absorption spectra is at 360 nm, and the particle size is 20 xcexcmxcx9c50 xcexcm. Pulverized particles prior to powdering may be used as they are.
It will be noted that if there are some commercially available materials which considerably lower in their performance on absorption of moisture, they may be used after encapsulated in glass or a transparent organic polymer resin such as polycarbonate, or may be used by incorporation in a substrate or by attachment on the surface of a substrate.
When a photocatalytic body is made of a mixture of the particles of a spontaneous light-emitting ceramic or a light storage-type light-emitting ceramic or molded particles obtained by mixing the fine particles of these ceramics (hereinafter referred to as mixed particles) with a photocatalyst, the photocatalytic semiconductor of the photocatalytic body is excited by means of UV light radiated from the spontaneous light-emitting ceramic particles or generated by consumption of the energy accumulated in the particles of the light storage-type light-emitting ceramic. Thus, the photocatalytic function is continued if the UV irradiation against the photocatalytic body is interrupted. Moreover, the particles of the spontaneous light-emitting ceramic or the light storage-type light-emitting ceramic usually emanates green, blue or orange-colored visible light, which may be utilized for decoration or directional sign in the dark.
When the photocatalytic semiconductor is controlled in its composition (by addition of inorganic pigments or metals), or is controlled in thermal treatment during the course of the preparation, it can be possible to change a wavelength (absorption band) of UV light necessary for showing the catalytic function, i.e. an excitation wavelength. For instance, if CrO3 is added to TiO2 in small amounts, the absorption band is shifted toward a side of a longer wavelength. This permits the photocatalytic body to be in coincidence with the emission spectral characteristics of a spontaneous UV radiating material or a light storage-type UV radiating material. Proper choice of a photocatalytic semiconductor in coincidence with a wavelength of UV light to be applied thereto becomes possible.
In contrast, the emission spectral characteristics of a spontaneous UV radiating material or a light storage-type UV radiating material may be brought into coincidence with the excitation wavelength of a photocatalytic semiconductor. For instance, the excitation wavelength of titanium oxide is in the range of 180 nmxcx9c400 nm. Light storage-type UV radiating materials responsible for the wavelength have never been commercially available.
Light storage ceramics which are commercially available and permit afterglow over a long time include xe2x80x9cLuminovaxe2x80x9d series of NEMOTO and CO., LTD, with some of the series having an afterglow time exceeding 1000 minutes. The light storage ceramics of the long-time afterglow are prepared by adding alumina to a starting main material such as strontium carbonate or calcium carbonate, further adding europium or dysprosium as an activator, and then adding an element such as of lanthanum, cerium, praseodymium, samarium, cadmium, terbium, holmium, erbium, thulium, ytterbium, ruthenium, manganese, tin and bismuth and boric acid as a flux, followed by thermal treatment at 1300xc2x0 C. The product obtained by this mixing procedure is a blue light emitter having a peak of the shortest wavelength of 440 nm.
In order to shift the emission wavelength to 400 nm or below which corresponds to the excitation wavelength of titanium oxide, additive metal elements may be added for causing the absorption wavelength of the xe2x80x9cLuminovaxe2x80x9d with a peak at 360 nm and the emission wavelength with a peak at 440 nm to come close to each other. Alternatively, if an emission wavelength of 440 nm or below does not generate on the emission of blue light at approximately 450 nm which is a phosphorescent wavelength characteristic inherent to minerals such as strontium, potassim and borax, a mineral element, which does not emanate any phosphorescent color, is shorter in wavelength than strontium, and has an emission wavelength of 400 nm or below without development of any color, may be purified and formulated to develop a light storage-type UV radiating material.
The photocatalytic semiconductor may be preliminarily supported on only the surfaces of unit particles, or may be supported on the entire surface of a molding after mixing of unit particles with the particles of a spontaneous light emitting ceramic or a light storage ceramic or the mixed particles and molding the mixture. In the former case, little photocatalytic semiconductor is deposited on the surfaces of the particles of a spontaneous light emitting ceramic or a light storage ceramic or the mixed particles, so that the quantity of UV light radiated from these particles becomes greater. With the particles of the light storage-type ceramic particles, UV light from outside can be efficiently absorbed.
The photocatalytic body may be admixed with photocatalytic function-assisting additive metals (Pt, Ag, Rh, RuO, Nb, Cu, Sn, NiO and the like) during the course of its preparation. These additives are well known as facilitating the photocatalytic reaction.